The present invention relates to spark plugs for use in automotive vehicles, and more particularly the invention relates to an improved spark plug having a specially shaped center electrode discharging face for controlling the path of discharge so as to ensure improved ignition performance.
In a known type of spark plug in which a center electrode and a ground electrode have their parallel plane surfaces disposed to oppose each other, when the high voltage produced by an ignition coil of the current interruption type is applied to the center electrode to produce an electric discharge, the discharge tends to start at its edge portion and consequently the path for the discharge to develop and travel tends to be directed from the edge portion (the side) of the center electrode toward the ground electrode. Also the location of discharge at the edge portion irregularly changes each time. As a result, the manner in which the air-fuel mixture is ignited and the resulting flame spreads varies in dependence on the path of discharge.
Under unfavorable conditions, such as, at low engine temperatures, low engine speeds, retarded ignition timing or the like where the mixture is not atomized satisfactorily and the flow velocity of the mixture is low, even if the mixture is ignited, the flame will be extinguished midway due to the obstruction and cooling by the ground electrode. In other words, under varying combustion conditions and unfavorable engine conditions, the engine will cause misfire and also deteriorated engine feeling and heat-melting loss of the exhaust emission controlling catalyst will be caused. Another type of spark plug is known in the art in which the ground electrode is formed with a U-shaped groove in the lengthwise direction, and this known spark plug has the similar deficiencies due to the cooling effect in the lengthwise direction of the ground electrode.